The state of application of atom probe tomography to electronic materials is assessed. The benefits and challenges of the technique are discussed with regard to its impact on this field of materials science. Specimen preparation in particular is emphasized as the key to success with modern atom probes. Electronic materials referenced in this paper include components of complementary metal/oxide/semiconductor (CMOS) structures, compound semiconductors, and thin films for data storage and general applications. Many examples from recent work are provided as illustrations of the types of information that can be derived and the impact this information can have on the research, development, processing, and failure analysis of electronic materials.
The performance of the pulsed-laser atom probe can be limited by both instrument and specimen factors. The experiments described in this article were designed to identify these factors so as to provide direction for further instrument and specimen development. Good agreement between voltage-pulsed and laser-pulsed data is found when the effective pulse fraction is less than 0.2 for pulsed-laser mode. Under the conditions reported in this article, the thermal tails of the peaks in the mass spectra did not show any significant change when produced with either a 10-ps or a 120-fs pulsed-laser source. Mass resolving power generally improves as the laser spot size and laser wavelength are decreased and as the specimen tip radius, specimen taper angle, and thermal diffusivity of the specimen material are increased. However, it is shown that two of the materials used in this study, aluminum and stainless steel, depend on these factors differently. A one-dimensional heat flow model is explored to explain these differences. The model correctly predicts the behavior of the aluminum samples, but breaks down for the stainless steel samples when the tip radius is large. A more accurate three-dimensional model is needed to overcome these discrepancies.
The fungal cell wall is a critically important structure that represents a permeability barrier and protective shield. We probed Candida albicans and Cryptococcus neoformans with liposomes containing amphotericin B (AmBisome), with or without 15-nm colloidal gold particles. The liposomes have a diameter of 60 to 80 nm, and yet their mode of action requires them to penetrate the fungal cell wall to deliver amphotericin B to the cell membrane, where it binds to ergosterol. Surprisingly, using cryofixation techniques with electron microscopy, we observed that the liposomes remained intact during transit through the cell wall of both yeast species, even though the predicted porosity of the cell wall (pore size, ~5.8 nm) is theoretically too small to allow these liposomes to pass through intact. C. albicans mutants with altered cell wall thickness and composition were similar in both their in vitro AmBisome susceptibility and the ability of liposomes to penetrate the cell wall. AmBisome exposed to ergosterol-deficient C. albicans failed to penetrate beyond the mannoprotein-rich outer cell wall layer. Melanization of C. neoformans and the absence of amphotericin B in the liposomes were also associated with a significant reduction in liposome penetration. Therefore, AmBisome can reach cell membranes intact, implying that fungal cell wall viscoelastic properties are permissive to vesicular structures. The fact that AmBisome can transit through chemically diverse cell wall matrices when these liposomes are larger than the theoretical cell wall porosity suggests that the wall is capable of rapid remodeling, which may also be the mechanism for release of extracellular vesicles.
The first dedicated local electrode atom probes (LEAP [a trademark of Imago Scientific Instruments Corporation]) have been built and tested as commercial prototypes. Several key performance parameters have been markedly improved relative to conventional three-dimensional atom probe (3DAP) designs. The Imago LEAP can operate at a sustained data collection rate of 1 million atoms/minute. This is some 600 times faster than the next fastest atom probe and large images can be collected in less than 1 h that otherwise would take many days. The field of view of the Imago LEAP is about 40 times larger than conventional 3DAPs. This makes it possible to analyze regions that are about 100 nm diameter by 100 nm deep containing on the order of 50 to 100 million atoms with this instrument. Several example applications that illustrate the advantages of the LEAP for materials analysis are presented.
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